The Earth's own magnetic field is probably created in the Earth's core, believed to contain molten iron, by a "fluid dynamo". These are rather special conditions and scientists who studied the Earth's dynamo in the 1950s and 1960s must have wondered whether the Earth's planetary magnetism was unique.

We now know better: space probes have found that Jupiter, Saturn, Uranus and Neptune all have magnetic fields, as does tiny Mercury. The Moon has patches of magnetized rocks and might have had a field when those rocks formed long ago, abd Venus seems non-magnetic. Mars was a mystery until September 1997, when Mars Global Surveyor found it to be magnetized in patches, like the moon but several times more strongly.

The sources of other planetary fields seem quite different from the Earth's and one can only speculate about their origin. For instance, does the magnetism of Jupiter and Saturn originate in cores of metallic hydrogen, a form which can exist under the enormous pressures at their centers? And since Uranus and Neptune do not generate enough pressure for hydrogen to become metallic--are their internal currents carried by ions dissolved in water or methane ices?

The magnetization of Mercury, Mars and the Moon must belong to a different class (see "Mercury: the Forgotten Planet" by R.M.Nelson, Scientific American, November 1997, p. 56). In particular, they may contain permanently magnetized rocks, from lavas which poured out in the distant past, when the parent body was magnetized, and became weakly magnetized themselves (this process does happen on Earth). All this, though, is speculation: we really do not yet know. The "Messenger" spacecraft, currently on its way, is due to reach Mercury in 2008 and attain an orbit around it in 2011.

As for the rest of the solar system: Venus appears to be unmagnetized, and the solar wind penetrates all the way to an "ionopause" above its dense atmosphere. In September 1997 Mars was found by "Mars Global Surveyor" to a weak magnet--just magnetized in patches, though some of them were moderately intense. Tiny Mercury, visited three times by Mariner 10 in 1974-5, is also magnetic, although its magnetosphere is so small that long-term trapping probably does not occur in it. Mariner 10 did see on its night side what appeared to be an abrupt acceleration event, perhaps similar to a substorm. The "Messenger" spacecraft, currently on its way, is due to reach Mercury in 2008 and attain an orbit around it in 2011.

Jupiter

Jupiter is the largest planet of the solar system and has the most powerful magnetic field, also the largest radiation belt; radio emissions from its belt were first detected by radio astronomers in 1955. Jupiter's magnetosphere was explored by the space probes Pioneer 10 and 11, Voyager 1 and 2, and Ulysses, and the space probe Galileo has been orbiting there since late 1995.

Jupiter's radiation belt is quite intense, and just one pass through its denser part by Pioneer 10 in 1973 was enough to cause some radiation damage, luckily rather minor.

Differences in Scale

The magnetospheres of the giant planets differ from the Earth's in at least four ways. First, they are much bigger, not only because the planetary magnets are stronger but also because the solar wind weakens as it moves away from the Sun and spreads out. Both of these factors cause the solar wind to be stopped further away from the planet than is the case with Earth.

The speed of the solar wind however remains the same, about 400 km/sec. As a result, the wind needs a much longer time to traverse the length of the magnetosphere.

With the Earth's magnetosphere, it takes the solar wind about one hour to advance from the "nose" to the distant tail regions where ISEE-3 and Geotail have probed it, some 200 RE downstream. During that one hour the Earth rotates by a rather small angle, 15 degrees, and if "open" field lines in the lobes connect it to the solar wind, those lines might become twisted by about 15 degrees.

If Jupiter's magnetosphere has the same proportions, the solar wind would need 2-3 days to cover the corresponding distance (equal to about half the Earth-Sun distance!), during which the planet might have rotated 5-7 times around its axis. One might therefore expect the lobes of Jupiter's magnetotail (and Saturn's, too) to be severely twisted, and the Galileo mission might be the first opportunity to examine this point. All other probes sent to Jupiter used the planet as a pivot to gain extra speed, the way "Wind" used the moon, and the orbits required for this maneuver kept them out of the lobes.

Satellites

Secondly, all these planets possess satellites and rings within their radiation belts (all four have rings, but only Saturn's ring is big enough to be easily seen from Earth). These absorb some of the trapped ions and electrons and produce dips in the profiles of the belts.

But they do more than that. Saturn seems to have an inner belt like the Earth's, and calculations suggest it is produced by cosmic ray neutrons ejected from the planet's rings. Jupiter's magnetosphere is heavily loaded with sulfur ions, believed to originate in the sulfur volcanoes of the satellite Io. This may also be the source of the sodium cloud around the planet, studied by telescopes from Earth.

Rotation

A third difference is the role of planetary rotation. The Earth is surrounded by a cloud of cool plasma--essentially, the upwards continuation of the ionosphere--which extends to about 5 Earth radii (the distance varies) and which rotates with the Earth.

The planets with the largest magnetospheres, Jupiter and Saturn, rotate rapidly (periods of about 10 hours), and data from space probes has suggested that the plasma surrounding them participates in that rotation to a much greater extent than the Earth's, perhaps up to the "nose" itself. How then do intense radiation belts arise? Perhaps very powerful magnetic storms overcome the rotation and inject them deep into the magnetosphere, or perhaps the process differs from what occurs near Earth. Again, Galileo might tell.

Tilt

Finally, there exist differences in the tilt of the magnetic axis. Earth has a magnetic axis inclined by 11.2 degrees to its rotation axis, which itself is inclined by 23.5 degrees to the direction perpendicular to the plane of the Earth's orbit; that plane also contains the direction from which the solar wind arrives. The upshot is that the Earth's magnetic axis is usually almost perpendicular to the solar wind, with either pole nodding periodically towards the Sun with an angle which is at most just under 35 degrees (11.2 + 23.5). Therefore, our average view of the magnetosphere, and most pictures in these files, draw the Earth's magnet as perpendicular to the solar wind.

Jupiter's magnetic axis is inclined to its rotation axis by about the same amount as the Earth's. Its magnetic north-south polarity is the opposite of the Earth's--but it's worth noting that fossil magnetic records, in sea-floor rocks, indicate that the Earth's polarity has reversed many times in the distant past. Saturn's magnetic axis seems exactly aligned with its rotation axis, within the errors of the observations, and that has bothered some theorists, since a 1931 theorem by Thomas Cowling stated that a planetary dynamo field cannot be axially symmetric. However, since the magnetic fields of irregularities die out quickly with distance, it can be that observations closer to the planet might find an asymmetry.

A "head-on" magnetosphere.

The real surprise came with Uranus, whose rotation axis is nearly parallel to its orbital plane. At the time of the 1986 fly-by of Voyager 2, that axis pointed almost exactly at the Sun. Based on their experience with Earth, Jupiter and Saturn, scientists expected the magnetic axis of Uranus to be close to its rotation axis, and to also point more or less sunward. They therefore expected a completely different magnetosphere, a "head-on" magnetosphere which met the solar wind not with a "hard" nose of ordered magnetic field lines (as the Earth does), but with its "soft" cusp region (picture on the left). Earth never attains this position.

But it wasn't to be. As Voyager 2 found, the magnetic axis of Uranus was actually steeply inclined to its rotation axis, at nearly 60 degrees, causing it to spin around like the axis of a top that is about to topple. As a result, the direction of the magnetic axis in space varied constantly and rapidly, but it never pointed towards the Sun--though it might do so, briefly, in other parts of the planet's orbit. Neptune was somewhat similar, with its magnetic axis angled by 47 degrees to its rotation axis.

All this suggests that not only isn't the Earth's magnetosphere unique, but different kinds of magnetospheres are possible, and some of them can be found in our solar system. Not only do we have in our magnetosphere a natural laboratory for studying cosmic plasmas, but different examples of such plasmas are also accessible (though not easily), to be studied perhaps by future generations. We are indeed fortunate!